The smallest nuclear weapon the US produced was the "Davy Crockett" -
a recoilless rifle round. It weighed about 51 pounds, was 16 inches long and 11 inches in
diameter. It produced a variable yield of up to 1 kiloton.

The Soviets supposedly produced "suitcase nukes" and there is no
reason to doubt this assertion. Former Soviet General Ledbed has asserted that a number of
these are not accounted for. There are reasons, however, to doubt his assertions given his
political position. Interestingly, Ledbed was recently killed in a helicopter crash.

The Soviets supposedly produced "suitcase nukes" and there is a US
DOE estimate that only 4kg of Plutonium is necessary to make a fission weapon. Some
believe that only 1kg is needed.

North Korea's weapon's program is using an implosion design with Plutonium. This design may eventually lead to concealable nuclear weapons, which might be sold to terrorists or terrorist supporting countries for anonymous nuclear attacks.

Basic Nuclear Weapons Engineering

All known nuclear weapons require the fission of Uranium (235 or 233) or
Plutonium 239. An "atomic bomb" (fission weapon) uses the fission energy
directly, while a "hydrogen bomb" (thermonuclear weapon) uses fission to ignite
a fusion reaction, achieving much higher energy release. In theory, there is no limit to
the power of a fusion bomb. There has been speculation that it is possible to create
useful fusion weapons without a uranium trigger, but no reliable unclassified information
indicates that this is true, and there is a difficult physical principle to overcome.

Uranium consisting of unnaturally high amounts of isotopes 233 and 235 is
called enriched uranium. Uranium is a common element on earth, but U-233
and U-235 constitute small percentages of it (<1%) and are always found mixed with the
less useful U-238.

Fission is the process whereby an atom's nucleus splits,
releasing a large amount of energy. In a simple fission weapon, fission occurs when a
neutron is absorbed by a nucleus, causing it to be highly unstable, at which point the
nucleus splits, releasing a large amount of energy and more neutrons. This only occurs
easily in a few isotopes (in Uranium, 235 or 233).

A chain reaction occurs because the fission of a nucleus
releases additional neutrons, which can then cause fission in more nuclei.

Critical mass is the amount of fissile material needed for a
chain reaction to become self-sustaining. This means that for each neutron released in the
material, on average one more neutron will be released as a result. The critical mass is a
factor not just of the type and amount of material, but also its instantaneous density and
geometry. In other words, a mass of plutonium might not be critical until it is rapidly
and highly compressed by high velocity "implosion."

Fission weapons explode when the fissile material is suddenly placed in a
configuration significantly greater than the critical mass - a state of supercriticality.
If critical mass is reached too slowly, the weapon will explode with greatly reduced
energy, possibly simply melting.

When a weapon has reached supercriticality, it still may not explode unless a
neutron passes into the core. Since a high explosive implosion maintains criticality for
only a few microseconds, a neutron flux generator may be required to guarantee this.

The easiest weapon to build uses a large amount (tens of kilograms) of
enriched uranium. Because uranium releases neutrons at a very low rate, the weapon can use
a relatively long "assembly time" to reach supercriticality. One design uses a
sphere with a cylindrical hole in it, and a "gun" to fire a cylinder of uranium
into that hole. Until the cylinder is inserted, both assemblies are well below critical
mass, but when the cylinder is inserted, the mass rapidly rises to supercriticality. A
neutron randomly released by the material during this process triggers the chain reaction.
This weapon is so simple that the US used one against Nagasaki without ever testing the
design. These weapons tend to be fairly large and inefficient, although the design was
used in a US nuclear artillery shell.

A plutonium based weapon cannot use the "gun" approach, because
plutonium releases too many neutrons, which would cause the chain reaction to start long
before the mass was supercritical enough to cause a large explosion. Hence plutonium
weapons require assembly by compressing a sphere or shell of plutonium very rapidly, using
high velocity explosives. This neccessitates very high quality explosives, a very precise
machining of all parts, and an electrical detonating system which can deliver very high
energy pulses to a number of detonators with great timing precision. Hence plutonium based
weapons are significantly harder to build. The US tested one at Trinity Site before
deploying another against Nagasaki.

A uranium weapon can also use the implosion approach, to achieve greater
efficiency. However, in this case it requires a neutron flux generator to assure that
enough neutrons flood the core during the maximal period of compression that the chain
reaction will start and the weapon will be efficient.

Uranium weapons require the production of enriched uranium (although it does
not have to be as highly enriched as is used in some power reactors). This is a complex
process because it requires the separation of isotopes of the same element (which means
they have the same chemical behavior) and the isotopes have almost the same weight. Thus
multiple stages of gas diffusion, centrifuges or electromagnetic (calutron) separation are
required. This is inevitably a major industrial project, and is likely to be detectable by
intelligence agencies. However, the centrifuge method can be distributed, making it
hard to spot. It is believed that Saddam Hussein intended to use this approach once he was
rid of UN inspectors. Laser separation has also been used. In addition, there may be new
technologies that make enrichment much easier to do or at least easier to conceal. Only a
ton or so of natural uranium is required to produce enough entriched uranium for a weapon.

Plutonium weapons require the production of highly pure plutonium. Because
plutonium is a not found in nature, it must be made in a nuclear reactor. Once made,
however, it is relatively easy to extract because it is chemically different from the
other elements in the mix, although the extraction process must take place in an
environment made extremely radioactive by other elements mixed with the plutonium.

Fusion weapons can be much more powerful than fission weapons, but
require a subtle and difficult design. However, at least two different teams (Teller-Ulam
and Sakarov) independently discovered the same approach. Since that time knowledge of
fusion designs has spread through espionage and possibly technology trades. The details of
fusion weapons will not be discussed here - see the Nuclear Weapons FAQ for
vastly more information.

Radiation Risk - The Facts, not the Scare Stories

Myth:
Nuclear war would end human life on earth through radiation. Fact:If all
of the nuclear weapons in stock at the height of the cold war were detonated, the average
radiation dose per person is only 1/100th of a lethal dose, and well below doses which can
be shown to have even long term effects (such as cancer).

Myth:
Chernobyl caused or will cause thousands of deaths. Fact:The Chernobyl
disaster was the worst possible reactor disaster. It released an extremely large amount of
radiation into the environment (see below). Even so, there have been no detectable
increases in death rates even among the most highly exposed population (other than those
who received extremely high doses fighting the fire, and many of whom died as a result).
The radiation levels of the "highly radioactive" regions
evacuated after the event are significantly lower than the natural
radiation level in many parts of the world. Long term very sensitive genetic studies of
animals in the most highly exposed region have found no abnormalities. There is no excess
of three eared rabbits or 10 pound cockroaches around Chernobyl!

Myth:Fallout
caused deaths in Japanese nuclear bombings. Fact:There was no significant
fallout in the vicinity of the Hiroshima and Nagasaki bombings. All radiation injuries
were a result of immediate (first 1 minute) radiation.

The United
States Transuranium and Uranium Registries (USTUR),
operated by Washington State University, reports: "The
health effects from plutonium, americium, and uranium intakes by humans, as
determined with USTUR data can be summarized in two words, virtually none.A study of the causes of death of USTUR organ
donors has been completed.The study showed
that the vast majority of USTUR donors died from the same diseases that have caused the
deaths of most of the U. S. population, heart disease, strokes, and cancers not
necessarily associated with radiation exposure.This
is in spite of the fact that the USTUR donors are a biased population in that a number of
donors volunteered for the program after having been diagnosed with cancer.The average age at death of USTUR registrants is
63 years (range between 25 and 91 years).The
average age of USTUR registrants who are still living is 73 years (range between 30 and 93
years)."

The only
human cases of significant fallout exposure to humans (as of 1964, and outside of the
USSR) were in the Marshall Islands, after a U.S. test (CROSSROADS, Bikini Atoll,
1946). The short term effects were skin burns, nausea, and other symptoms typical of
exposure to high radiation doses. Even so, there was only one cancer (leukemia) likely
caused by the radiation, 18 years after exposure of a 1 year old. Of the pregnancies in
progress at time of exposure, there was one miscarriage (no evidence for or against
radiation relationship). The rest produced healthy children. Not surprisingly, there were
a large number of cases of thyroid problems, which lead to some reduced growth in
children. A study of the 40,000 military members who participated found no scientific
evidence of radiation induced cancers. References are here.
These results do not mean that fallout is harmless - far from it, but they show that even
radiation intense enough to produce burns and nausea need not create a significantly
increased risk of cancer.

Almost all
radioactivity in fallout - even in a ground burst - comes from the fission products
themselves or transmutation of parts of the weapon. Thus air bursts and ground bursts
produce approximately the same amount of radioactive products. However, ground bursts
cause much more of the radioactive debris to be deposited within a fallout pattern, rather
than distributed (and accordingly diluted and decayed) across the entire planet.

There is
evidence that radiation is beneficial and improves health (radiation
hormesis) up to some surprisingly high levels..

More
Technical Information

Blast
Effects

The blast
effect is primarily determined by the "overpressure" - given in english units in
PSI.

This
effect at any distance is proportional to the cube root of the weapons yield. Thus a 20
megaton bomb, which is large by today's standards, will affect only 10 times the radius of
a 20 kiloton bomb - which was the yield at Hiroshima.

In
Hiroshima, there was a 50% survival rate .12 miles (200 meters) from ground zero. The bomb
went off at 1850 feet above ground zero with a yield of about 20kt. Concrete structures at
ground zero survived.

In
Hiroshima, there was only one known case of burst eardrums among the survivors.

A human
being can withstand up to about 35PSI of peak overpressure from a nuclear blast (1%
fatality rate). Your mileage may vary. Thus a human will almost always survive the
blast overpressure at approximately the following distances (slant range) from a blast
according to the following table:

Distance From Blast to
Survive Blast Wave

Yield

Distance (mi)

Distance (km)

Comments

20 kT

.35

.56

Hiroshima and Nagasaki

600 kT

1.1

1.8

Typical Strategic US Nuke

20 MT

3.5

5.6

Very Big Bomb

The blast
wave can, however, pick people up and throw them. For a 165 pound standing person to be
thrown at 20 feet per second, the following table shows the distance from the blast:

Distance From Blast to
be Thrown at 20 fps

Yield

Distance (mi)

Distance (km)

20kT

1.06

1.75

600kT

4.1

6.6

20MT

16.8

27

Max Wind at Distance
from Blast

Yield

1 mi
1.6 km

3 mi
4.8 km

10 mi
16 km

30 mi
48 km

20kT

200 mph
89 mps

47 mph
21 mps

5 mph
2 mps

~0

600kT

1000 mph
447 mps

210 mph
94 mps

51 mps
23

5 mps
2

20MT

off scale

1200 mph
536 mps

195 mph
87 mps

47 mph
21 mps

The greatest
danger from the blast wave comes from destruction of structures and the conversion of
objects into missiles. The following tables gives the destruction distance from various
yields for a few kinds of structures:

Window Breakage

Yield

Distance (mi)

Distance (km)

20kT

3.2

5.1

600kT

10

16

20MT

32

51

Wood-frame Building
Destruction

Yield

Distance (mi)

Distance (km)

20kT

1.5

1.9

600kT

4.8

7.7

20MT

15

19

Multi-story Brick

Yield

Distance (mi)

Distance (km)

20kT

1.

1.6

600kT

3.0

4.8

20MT

10

16

Multi-story Reinforced
Concrete Offices

Yield

Distance (mi)

Distance (km)

20kT

.5

.81

600kT

1.3

2.1

20MT

5

8.1

A ground
burst produces a crater. The following table shows crater sizes:

Crater Sizes

Yield

Width
(feet)

Width
(m)

Depth
(feet)

Depth
(m)

20kT

633

193

80

24

600kT

2112

643

211

64

20MT

7392

2253

792

241

Historic Radiation Releases

Historic Radiation
Releases (MegaCuries)

Chernobyl

7.3
MCi

Hiroshima

1.4
MCi

Hanford (I-131 only)

0.74
MCi

Three Mile Island

0.000015
MCi

Nuclear
Physics Reference

The negative effects of radiation can be divided into immediate effects
(from very high dosages) and long term effects (from lower dosages). Long term effects are
thought to be either the development of cancer, or genetic damage passed on to offspring.
However, there is no evidence that low to moderate levels of radiation cause any long term
damage in humans, and some evidence that it may be beneficial (radiation hormesis). Both
long term effects would be a result of damage to DNA - most likely nuclear DNA (as opposed
to mitochondrial DNA). However, the human cell experiences an average of 70 million
(7x10^7) DNA damages per year. Only 5 of these are attributable to natural radiation. Even
radiation much higher than natural radiation would produce a negligible percentage of the
total DNA damage. The average natural human dose is 2.2 mSv per year (see below for
units). The lethal dose is typically 3000 mSv - 5000 mSv.